You finally meet the Mendez brothers, both nasty pieces of work. They want you to prove you weren't the one who stole their cocaine by providing evidence that it was Martinez.
Head down to Washington Beach police station and watch Martinez come out with the DEA officer. When they get in the car drive near to the vantage point then get out and walk to the marker. Zoom the camera in to get a good close up shot of Martinez with the DEA.
Get back in your car and follow the DEA officer's car to the marina. Step into the marker here and zoom in for a close up picture of Martinez on the boat.
Get back in your car (don't take the DEA's car) and drive to Lance's mansion to pick up Forbes' ID. You'll have a four-star wanted level by this point, so once you have the ID get to a Pay 'n' Spray.
Once you've lost the heat head over to the Print Works. When you get there Lance will use the ID to make a fake ID for Jerry Martinez, making him into an undercover cop, just the evidence you need to keep the Mendez Brothers on your side.